A single pass electrophotographic printer typically employs four image forming stations, each one responsible for printing one of four primary colors, typically cyan, magenta, yellow, and black. The individual images, known as separations, produced by each of the four image forming stations are combined to produce the final output image. In tandem print engines, the four image forming stations are aligned in the paper transport direction such that each separation is formed in succession on the copy sheet as the copy sheet is transported through each print station. Typically, a belt transports the copy sheet. In some printers, a belt serves as an intermediate transfer member (ITM). The image forming stations transfer the individual image separations onto the ITM to form a composite image on the ITM. The composite image is then transferred from the ITM to the copy sheet at a transfer station.
The alignment of the image separations produced by each image forming station is critical to producing a quality printed image. Various factors affect the proper alignment of the image forming stations, such as tolerances, wear, and thermal expansion/contraction. It can be expensive and impractical to control tolerances and wear in order to provide acceptable color registration. Therefore, many printers include a mechanism to detect and correct for color registration errors.
One technique used to detect color registration errors is to print a set of overlapping registration marks in two or more different colors and examine the result. Registration errors are detected by misalignment of the registration marks. Some registration patterns, exemplified by U.S. Pat. No. 6,408,156, are designed to indicate the degree of misregistration. In the '156 patent, the registration pattern comprises a series of bars printed in two different colors, one of which is usually black and the other of which is usually yellow, magenta or cyan. The bars of one color have varying degrees of offset from corresponding bars of the other color. The degree of misregistration is indicated by the set of bars that align.
In most prior art printers, detection of registration errors is performed manually by the user. The registration pattern is printed on an output sheet and the user visually examines the registration pattern to determine the amount of any registration errors. After determining the amount of any registration errors, the user enters correction data into the printer via the operator panel. Correction of registration errors by manual inspection of registration marks and manual entry of correction data into the printer is inconvenient for users.
Various methods for automatic detection and correction of color registration errors are known. One such method is to print the registration pattern on a transport belt or intermediate transfer belt, illuminate the registration pattern, and detect the amount of reflected light with an optical sensor. Optical detection works best when a black registration mark is printed over the top of a color (e.g. magenta, cyan, or yellow) registration mark on a dark belt to produce a composite registration pattern. When progressively greater amounts of the color toner are revealed from underneath a layer of black toner, the amount of light reflected by the color toner increases and is easily detected. Optical detection does not work as well when a color registration mark is printed over the top of a black registration mark on a dark belt because the black toner is closer in reflectance to the belt. Thus, revealing progressively larger amounts of black toner from underneath a layer of color toner produces little net difference in reflectance. The opposite is true if the paper transport belt or ITM belt is light.
In single pass electrophotographic printers, the order of the image forming stations affects optical sensing of registration patterns. Placing the black image forming station last is compatible with optical sensing of registration patterns on a dark belt. In some cases, however, it may be desirable to place the black image forming station first, making optical sensing of the registration patterns problematic. This problem may be solved by using a specular sensor. In general, black toner has a diffuse reflectance characteristic compared to the surface of the ITM belt, which is mostly specular. Thus, a specular sensor can be used to sense the loss of specular signals as black toner covers the bare belt. The use of a specular sensor for color registration, however, is undesirable because it may require separate sensors for color registration and toner density correction.